The 3rd-Generation Partnership Project (3GPP) has developed specifications for a fourth-generation wireless communications technology known as “Long Term Evolution,” or “LTE.” LTE uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and DFT-spread OFDM in the uplink, where DFT denotes “Discrete Fourier Transform”. The basic LTE physical resources can thus be seen as a time-frequency grid, as illustrated in FIG. 1, where each resource element corresponds to one subcarrier during one OFDM symbol interval on a particular antenna port. An antenna port is defined such that the channel over which a symbol on the antenna port is conveyed can be inferred from the channel over which another symbol on the same antenna port is conveyed. There is one resource grid per antenna port.
In the time domain, LTE downlink transmissions are organized into radio frames of ten milliseconds. Each radio frame includes ten equally sized subframes of one millisecond. FIG. 2 illustrates this arrangement one sees from the diagram that each subframe is divided into two slots, with each slot having a duration of 0.5 milliseconds.
Resource allocation in LTE is described in terms of “Physical Resource Blocks,” or “PRBs.” As shown in FIG. 3, a PRB corresponds to one slot in the time domain and twelve contiguous 15-kHz subcarriers in the frequency domain. The bandwidth, NBW, of the overall system determines the number of PRBs in each slot, and each PRB spans six or seven OFDM symbols, depending upon the length of the cyclic prefix (CP) used. Two consecutive PRBs in time represent a PRB pair. User scheduling by the LTE base station, referred to as an “eNodeB” or “eNB”, is generally performed using the PRB pair as the smallest unit of resource allocation.
Transmissions in LTE are dynamically scheduled based on transmitting downlink assignments and uplink grants to targeted mobile terminals (referred to as “user equipment,” or “UEs,” in 3GPP terminology). According to Release 8 of the 3GPP standards, which was the first release to include specifications for LTE, the downlink assignments and uplink grants are transmitted in a defined control region using Physical Downlink Control Channels (PDCCHs) targeted to specific UEs. The search space for PDCCH reception, which defines those resources in any given subframe that might include control information for the UEs, is known to the UEs. The UEs thus blindly decode those portions of the received signal to find PDCCHs targeted to them.
More broadly, PDCCHs are used to convey UE-specific scheduling assignments for the downlink and uplink grants, as noted, and are further used for Physical Random Access Channel (PRACH) responses, uplink power control commands, and common scheduling assignments for signaling messages that include, among other things, system information and paging.
FIG. 4 illustrates that a “normal” downlink subframe includes a control region at the beginning of the subframe, followed by a data region. The size of the control region in which PDCCHs are transmitted can vary in size from one to four OFDM symbols in dependence on the involved configuration. A Physical Control Format Indicator (PCFICH) is used to indicate the control region length and is transmitted within the control region at locations known by the UEs. A UE thus learns the size of the control region in a given downlink subframe by decoding the PCFICH transmitted in that subframe, and therefore knows in which OFDM symbol the data transmission starts.
PDCCHs are made up of Control Channel Elements (CCEs), where each CCE consists of nine Resource Element Groups (REGs). Each REG in turn consists of four resource elements (REs). LTE defines four PDCCH formats 0-3, which use aggregation levels of 1, 2, 4, and 8 CCEs, respectively. Given the modulation format used for PDCCH transmission, two bits can be transmitted on each individual RE aggregated within a PDCCH; with 1 CCE=9 REGs=36 REs and 2 bits/symbol, one can transmit 72 bits via a format 0 PDCCH, 144 bits via a format 1 PDCCH, etc. As noted, PDCCHs are transmitted in the defined control region—the first 1-4 symbols—of any given downlink subframe and extend over substantially the entire system bandwidth. Thus, the size of the control region in the given downlink subframe and the overall system bandwidth define the number of overall CCEs available for PDCCH transmission.
FIG. 4 also illustrates the presence of Cell-specific Reference Symbols (CRS) within the downlink subframe. The locations and values of CRS are known by the UEs, which use the received CRS for estimation of the radio channel. The channel estimates are in turn used in the demodulation of data by the UEs. CRS are also used for mobility measurements performed by the UEs.
Because the CRS are common to all UEs in a cell, the transmission of CRS cannot be easily adapted to suit the needs of a particular UE. Therefore, LTE also supports UE-specific reference symbols generally intended only for assisting channel estimation for demodulation purposes. These UE-specific RS are referred to as Demodulation Reference Symbols (DMRS). DMRS for a particular UE are placed in the data region of the downlink subframe, as part of Physical Downlink Shared Channel (PDSCH) transmissions.
Release 11 of the 3GPP standards introduced the enhanced PDCCH (ePDCCH) as an additional, and more flexible, channel for transmitting control messages to UEs. An ePDCCH uses resources in the data region associated with PDSCH transmissions, rather than resource elements within the defined control region at the beginning of the subframe. See “Universal Mobile Telecommunications System (UMTS); Technical Specifications and Technical Reports for a UTRAN-based 3GPP system”, 3GPP TS 21.101, v.11.0.0.
FIG. 5 provides a basic illustration of PRB pairs allocated from the data region of a downlink subframe, for use in the transmission of given ePDCCHs. The remaining PRB pairs in the data portion of the subframe can be used for PDSCH transmissions; hence the ePDCCH transmissions are frequency multiplexed with PDSCH transmissions. That arrangement differs from PDCCH transmissions, which are time multiplexed with respect to PDSCH transmissions—i.e., PDCCH transmissions occur only in the control portion of the downlink subframe, which occurs in time before the data portion in which PDSCH transmissions are performed.
Resource allocation for PDSCH transmissions can be according to several resource allocation types, depending on the downlink control information (DCI) format. Some resource allocation types have a minimum scheduling granularity of a resource block group (RBG). An RBG is a set of adjacent (in frequency) resource blocks. When scheduling the UE according to these resource allocation types, the UE is allocated resources in terms of RBGs, rather than according to individual resource blocks (RBs) or RB pairs.
When a UE is scheduled in the downlink from an ePDCCH, the UE shall assume that the PRB pairs carrying the downlink assignment are excluded from the resource allocation, i.e., rate matching applies. For example, if a UE is scheduled to receive PDSCH in a certain RBG that consists of three adjacent PRB pairs, and if one of these PRB pairs contains the downlink assignment, then the UE shall assume that the PDSCH is transmitted in only the two remaining PRB pairs in this RBG. Notably, Release 11 does not support multiplexing of PDSCH and ePDCCH transmission within the same PRB pair.
ePDCCH messages are made up of enhanced Control Channel Elements (eCCEs), which are analogous to the CCEs used in the PDCCH. For purposes of mapping ePDCCH messages to PRB pairs, each PRB pair is divided into sixteen enhanced resource element groups (eREGs). Each eCCE is made up of four or eight of these eREGs, for normal and extended cyclic prefix, respectively. An ePDCCH is consequently mapped to a multiple of either four or eight eREGs, depending on the aggregation level. The eREGs belonging to a particular ePDCCH resides in either a single PRB pair (as is typical for localized transmission) or a multiple of PRB pairs (as is typical for distributed transmission).
One example of the possible division of a PRB pair into eREGs is illustrated in FIG. 6, which illustrates an unconstrained subframe. Each block or tile in the figure is an individual resource element (RE) and the tile number corresponds to the EREG that the RE is grouped within. For example, tiles having the dotted background all belong to same EREG indexed at 0.
A UE can be configured so that multiple sets of PRB pairs are available for use as ePDCCH resources. Each ePDCCH resource set consists of N=2, 4, or 8 PRB pairs. In addition, two modes of ePDCCH transmission are supported, i.e., localized and distributed ePDCCH transmission. Each set of ePDCCH resources is independently configured as being of localized or distributed type. In distributed transmission, an ePDCCH is mapped to resource elements in an ePDCCH set in a distributed manner, i.e., using multiple PRB pairs that are separated from each other in frequency. In this way, frequency diversity can be achieved for the ePDCCH message. As of Release 11, the ePDCCH can be mapped to resource elements in up to D PRB pairs, where D=2, 4, or 8 (the value of D=16 is also being considered in 3GPP). FIG. 7A illustrates an example of distributed transmission, where D=4 is illustrated. As seen in the example, the ePDCCH is divided into four parts, which are mapped to different PRB pairs. These four parts may correspond to eCCEs, for example.
In a localized transmission, on the other hand, an ePDCCH is mapped to one PRB pair only, if the space allows. Mapping to a single PRB pair is always possible for aggregation levels one and two, and is possible also for aggregation level four for the case of a normal, “unconstrained” subframe and a normal CP length. Here, an “unconstrained” or normal subframe is one having a PDSCH region that is not abbreviated. Constrained subframes include “special” subframes in TDD LTE that include uplink and downlink portions, and subframes that are given over to another purpose, such as Multicast-Broadcast Single Frequency Network (MBSFN) transmissions. The number of eCCEs that fit into one PRB pair is given by Table 1, below. Thus, for example, in a normal subframe with a normal CP length, localized transmission using aggregation levels of 1, 2, or 4 uses only a single PRB pair, while localized transmission using an aggregation level of 8 requires the use of two PRB pairs.
TABLE 1Number of eCCEs per PRB pair in localized transmissionNormal cyclic prefixExtended cyclic prefixSpecialSpecialSpecialNormalsubframe,subframe,subframe,sub-configurationconfiguration 1,Normalconfiguration 1,frame3, 4, 82, 6, 7, 9subframe2, 3, 5, 642
In case the aggregation level of the ePDCCH is too large, a second PRB pair is used as well, and so on, using more PRB pairs, until all eCCEs belonging to the ePDCCH have been mapped. FIG. 7B illustrates an example of localized transmission. In this example, the same four parts of the ePDCCH are mapped to a single PRB pair.
As described above, certain downlink resources are made available for sending PDCCH and ePDCCH messages to the UE. However, a given UE is not targeted to receive control channel messages in every subframe. Further, the UE does not know in advance precisely where a control channel message will be located among the resources made available for the control channel messages. Thus, the UE must search for a control message that may not exist, in each of several possible locations for the message. The concept of a “search space” is used to define a range of possible locations for control messages, to keep the required amount of searching to a reasonable level.
For the PDCCH, Release 8 of the 3GPP specifications for LTE define a search space Sk(L) for each of the possible aggregation levels Lϵ{1, 2, 4, 8}. This search space is defined by a contiguous set of CCEs given by the following:(Zk(L)+i)mod NCCE,k  (1)where NCCE,k is the total number of CCEs in the control region of subframe k, Zk(L) defines the start of the search space. i is an index value that ranges according to i=0, 1, . . . , M(L)·L−1, where M(L) is a pre-determined number of PDCCHs to monitor in the given search space, which depends on the aggregation level. Table 2, which is reproduced from Table 9.1.1-1 of 3GPP TS 36.213, “Physical Layer Procedures (Release 8),” provides the values of M(L) for each of the possible aggregation levels L. Each CCE contains 36 QPSK modulation symbols.
TABLE 2M(L) vs. Aggregation Level L for PDCCHSearch space Sk(L)Number of PDCCHTypeAggregation level LSize [in CCEs]candidates M(L)UE-specific16621264828162Common41648162
It should be noted that with this definition, search space for different aggregation levels may overlap with each other, regardless of system bandwidth. More specifically, UE-specific search space and common search space might overlap and the search spaces for different aggregation levels might overlap. One example is shown below, in Table 3, where there are nine CCEs in total and very frequent overlap between PDCCH candidates.
TABLE 3NCCE,K = 9, ZK(L) = {1, 6, 4, 0} for L = {1, 2, 4, 8}, respectively.Search space Sk(L)AggregationTypeLevel LPDCCH candidates in terms of CCE indexUE- Specific1{1}, {2}, {3}, {4}, {5}, {6}2{6, 7}, {8, 0}, {1, 2}, {3, 4}, {5, 6}, {7, 8}4{4, 5, 6, 7}, {8, 0, 1, 2}8{0, 1, 2, 3, 4, 5, 6, 7}, {8, 0, 1, 2, 3, 4, 5, 6}Common4{0, 1, 2, 3}, {4, 5, 6, 7}, {8, 0, 1, 2},{3, 4, 5, 6}8{0, 1, 2, 3, 4, 5, 6, 7}, {8, 0, 1, 2, 3, 4, 5, 6}
As is also the case for PDCCH, the ePDCCH is transmitted over radio resources shared by multiple UEs. The enhanced CCE (eCCE) is introduced as the equivalent of the CCE for PDCCH. Like a CCE, an eCCE also has a fixed number of resource elements. However, the number of resource elements actually available for ePDCCH mapping is generally fewer than the fixed number, because many resource elements are occupied by other signals such as Cell-specific Reference Signals (CRS) and Channel State Information-Reference Signal (CSI-RS). Code-chain rate matching is applied whenever a resource element belonging to an eCCE contains other colliding signals such as the CRS, CSI-RS, legacy control region or in case of Time Division Duplexing (TDD), the Guard Period (GP) and Uplink Pilot Time Slot (UpPTS).
Consider the example in FIG. 8, where item 40 illustrates the PDCCH mapping. The PDCCH always avoids the CRS, so that a CCE always contains Tavial=36 available resource elements. In item 42, on the other hand, it is shown how an eCCE contains 36 resource elements nominally, but the number of available resource elements is reduced in the event that there are colliding signals. Hence, Tavail≤36 resource elements for ePDCCH. Since the colliding signals are subframe dependent, the value of Tavail becomes subframe dependent as well, and could even be different for different eCCEs, if the collisions impact on the eCCEs unevenly. It is noted that when the number of eCCEs per PRB pair is two (see Table 1), the nominal number of resource elements per eCCE is not 36, but instead is either 72 (for normal CP length) or 64 (for extended CP length).
As of Release 11 of the 3GPP standards for LTE, the ePDCCH supports only the UE-specific search space, whereas the common search space remains to be monitored in the PDCCH in the same subframe. In future releases, the common search space may be introduced also for ePDCCH transmission. The Release 11 standards specify that the UE monitors eCCE aggregation levels 1, 2, 4, 8, 16 and 32, with restrictions shown in Table 4 below, where nEPDCCH is the number of available resource elements for ePDCCH transmission in a PRB pair. In Table 4, distributed and localized transmission refers to the ePDCCH mapping to resource elements.
TABLE 4Aggregation levels for ePDCCHAggregatian levelsNormal subframesand special subframes,configuration 3, 4, 8, withηEPDCCH <104 and usingnormal cyclic prefixAll other casesePDCCHLocalizedDistributedLocalizedDistributedformattransmissiontransmissiontransmissiontransmission02211144222884431616884—32—16
In distributed transmission, an ePDCCH can be mapped to resource elements in up to D PRB pairs, where D=2, 4, or 8 (the value of D=16 is also being considered in 3GPP). In this way, frequency diversity can be achieved for the ePDCCH message. See FIG. 7A for a schematic example in which a downlink subframe shows four parts belonging to an ePDCCH which is mapped to multiple of the enhanced control regions known as PRB pairs, to achieve distributed transmission and frequency diversity or sub-band precoding.
As of September 2012, the 3GPP has not reached agreement as to how four or eight eREGs respectively should be grouped into the eCCEs. It is also an open question as to how the encoded and modulated symbols of an ePDCCH message are mapped to the resource elements within the resources reserved by its associated eREGs. Further, the number of blind decodes per aggregation level for ePDCCH has not yet been decided in the 3GPP standardization work. Likewise, how randomization of the search space for localized and distributed mappings is generated has not yet been decided, although it is clear that overlap between ePDCCH candidates of different aggregation levels will occur also for the ePDCCH, as is the case for the PDCCH.
Time-Division Duplex (TDD) operation in LTE systems presents additional challenges with respect to PDCCH and ePDCCH to PUCCH mapping. These challenges to PUCCH HARQ-ACK resource determination arise from the asymmetry between uplink and downlink. When there are more downlink subframes than uplink subframes, the one to one mapping used in Frequency-Division Duplex (FDD) mode cannot be reused, since PUCCH resources selected according to this approach will collide with each other across different downlink subframes. On the other hand, overall HARQ-ACK resource utilization should be considered, since the resources for PUSCH transmission will be reduced if excessive uplink resources are reserved for PUCCH HARQ-ACK transmission. The TDD PUCCH resource for HARQ-ACK transmission in response to legacy PDCCH has been specified in the technical standardization document 3GPP TS 36.213, “Physical Layer Procedures,” v10.6.0.
FIG. 9 provides an illustration of the allocation of PUCCH resources for PDCCH, in TDD mode. The illustrated example is for four downlink subframes (SF0, SF1, SF2, and SF3) and one uplink subframe (SF4). Therein the resource determination for HARQ-ACK multiplexing and HARQ-ACK bundling are similar and can be derived as specified in 3GPP TS 36.213 v10.6.0, “Physical Layer Procedures”. It can be seen that the PUCCH HARQ-ACK resources will be stacked firstly for the lowest eCCE index of the DCI that fall within the first one-third CCEs of the control region, across multiple subframes (from SF0 to SF3) (marked with diagonal shading). These PUCCH HARQ-ACK resources are followed by the DCIs belonging to the second one-third CCEs of the control region (shaded). Finally are the last one-third CCEs (marked with cross-hatching). The design philosophy is that when system load is low, the control region could be automatically reduced by the dynamic signaling of PCFICH, hence the PUCCH HARQ-ACK resource could be compressed to a continuous region.
TDD PUCCH resource determination for ePDCCH has not been resolved in 3GPP RANI yet, i.e., no concrete solution is provided. However, a separate design different from FDD is needed, just as for PDCCH. Due to the fundamental differences in resource structures, the current design for PDCCH cannot be reused for ePDCCH. For example, PDCCH is a common control region (first one to four OFDM symbols) for all UEs while ePDCCH is multiplexed in frequency with PDSCH in a UE-specific manner. Accordingly, techniques for TDD PUCCH HARQ resource allocation for the Enhanced Physical Downlink Control Channel (ePDCCH) in radio communication systems are needed.